Effluent from diamond mining operations rich in calcium (Ca) has transformed softwater tundra lakes in the Northwest Territories, Canada. Lakes downstream of the Dominion Diamond Corporation Ekati Mine have experienced marked changes in water chemistry and cladoceran community composition since establishment of the mine in 1998. The greatest changes have occurred at the sites closest to the effluent discharge, with [Ca] increasing from <1 to >30 mg·L−1 and corresponding increases in pH from <7 to >8. A split was identified in the cladoceran communities with Holopedium glacialis (a jelly-clad cladoceran tolerant of low [Ca]) generally dominating the cladoceran community at [Ca] < 2.5 mg·L−1, while in impacted lakes with [Ca] ≥ 2.5 mg·L−1, Daphnia longiremis and Daphnia middendorffiana (taxa with higher [Ca] requirements) were often dominant. In contrast, the three study lakes that did not receive mining effluent maintained stable and low [Ca] (mean [Ca] = 0.66 ± 0.06 mg·L−1 (SD)) throughout the monitoring period and have not experienced directional shifts in their cladoceran communities.